1. Field of the Invention
The present invention generally relates to a mobile communication system, and more particularly to a method and an apparatus for efficient scheduling of an uplink in a mobile communication system for communication using a plurality of sub-carriers.
2. Description of the Related Art
Orthogonal Frequency Division Multiple Access (OFDMA), which is attracting attention as next generation mobile communication multiplexing technology, has been adopted in the standards of Institute of Electrical Electronics Engineers (IEEE) 802.16, IEEE 802.20, etc. However, according to Orthogonal Frequency Division Multiplexing (OFDM), which is modulation/demodulation technology of OFDMA, because a power amplifier has a high Peak-to-Average Power Ratio (PAPR), an increase in input back-off of the power amplifier is necessary in order to prevent non-linear distortion of a signal. Then, a maximum transmit power is inevitably similarly limited. Therefore, OFDMA has a low power efficiency.
When employing OFDMA as downlink multiplexing technology, there is no big problem because the transmitter is located in a Node B having no limitation in power. However, when employing OFDMA as uplink multiplexing technology, the transmitter is located in a User Equipment (UE) having a relatively large limitation in power. Therefore, a maximum power of the UE is limited, and the Node B coverage is similarly reduced. Therefore, in Long Term Evolution (LTE), which is 4th Generation (4G) mobile communication technology of the 3rd Generation Partnership Project (3GPP) standard, a Single Carrier FDMA (SC-FDMA) is being discussed as uplink multiplexing technology.
FIG. 1 shows a typical SC-FDMA transmitter. In FIG. 1, an NTX number of encoded and modulated symbols 102 is converted to frequency domain signals by a Discrete Fourier Transform (DFT) block 110. Then, the frequency domain signals are mapped to allocated sub-carriers by a sub-carrier mapper 120, and are then converted to a time domain signal by an Inverse Fast Fourier Transform (IFFT) block 130 having a size of NFFT. Then, a Cyclic Prefix (CP) inserter 140 inserts a CP for eliminating inter-symbol interference into the time domain signal, thereby outputting an OFDM symbol. Then, the OFDM symbol is loaded onto a Radio Frequency (RF) signal by an RF unit (not shown) including a transmission filter and a power amplifier, and is then transmitted by the RF signal.
The SC-FDMA transmitter in FIG. 1 is different from an OFDMA transmitter in that the SC-FDMA transmitter additionally includes a DFT block 110, so a final output signal of the SC-FDMA transmitter is not a frequency domain signal, but is a time domain signal. Therefore, the signal input to the DFT block 110 of FIG. 1 has the same PAPR as that of the signal output from the IFFT block 130.
FIG. 2 is a graph illustrating a comparison between PAPRs of a Quadrature Phase Shift Keying (QPSK)-modulated SC-FDMA signal and an OFDMA signal when NFFT=1024 and NTX=256. The graph shows curves of Pr indicating a probability that an estimated PAPR might be larger than an initial value PAPR0 of the PAPR (PAPR>PAPR0). As noted from the graph, when Pr is 0.01, that is, when Pr (PAPR>PAPR0)=0.01, the SC-FDMA signal has a gain of at least about 3 decibels (dB) in view of the PAPR in comparison with the OFDMA signal. Therefore, the SC-FDMA signal can have a higher power efficiency than the OFDMA signal.
As described above, the SC-FDMA signal has a lower PAPR than that of the OFDMA signal, and thus has a gain of at least about 3 dB in view of the power efficiency in comparison with the OFDMA signal. However, in order to make the PAPR of the DFT input and the PAPR of the IFFT output be identical with each other, the sub-carrier mapping inevitably has a limitation. FIGS. 3A and 3B illustrate a sub-carrier mapping according to SC-FDMA.
That is, in order to obtain a gain in the power efficiency, mapping of all of the NTX DFT output symbols to consecutive sub-carriers is necessary, as shown in FIG. 3A, or inserting an (L−1) number of 0 (null or zero) symbols between allocated sub-carriers is necessary, as shown in FIG. 3B. The sub-carrier mapping shown in FIG. 3A is called Localized FDMA (LFDMA), and the sub-carrier mapping shown in FIG. 3B is called Distributed FDMA (DFDMA).
An SC-FDMA may cause waste of resources in an uplink scheduling by a Node B due to the SC-FDMA having the limitation in the sub-carrier mapping as described above. Therefore, a need exists for technology capable of efficiently allocating at least one sub-carrier to each UE through uplink scheduling in an SC-FDMA system.